1. Field of the Invention
This invention relates to exhaust gas after-treatment systems, and more particularly to apparatuses, systems and methods for determining the distribution of particulate matter on a particulate filter.
2. Description of the Related Art
Environmental concerns have motivated the implementation of emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter, nitrogen oxides, and unburned hydrocarbons. Catalytic converters implemented in an exhaust gas after-treatment system have been used to eliminate many of the pollutants present in exhaust gas; however, to remove diesel particulate matter, typically a diesel particulate filter must be installed downstream from a catalytic converter, or in conjunction with a catalytic converter.
A common diesel particulate filter comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter subsequently accumulates on the surface of the filter, creating a buildup which must eventually be removed to prevent obstruction of the exhaust gas flow. Common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, results from incomplete combustion of fuel and generally comprises a large percentage of particulate matter buildup. Various conditions, including, but not limited to, engine operating conditions, mileage, driving style, and terrain affect the rate at which particulate matter accumulates within a diesel particulate filter.
Accumulation of particulate matter typically causes backpressure within the exhaust system which can degrade engine performance. Particulate matter, in general, oxidizes in the presence of NO2 at modest temperatures, or in the presence of oxygen at higher temperatures. If too much particulate matter has accumulated when oxidation begins, the oxidation rate may reach a point that it causes an uncontrolled temperature excursion. The resulting heat can destroy the filter and damage surrounding structures. Recovery can be an expensive process.
To prevent potentially hazardous situations, it is desirable to oxidize accumulated particulate matter in a controlled regeneration process before it builds to excessive levels. To oxidize the accumulated particulate matter, temperatures generally must exceed the temperatures typically reached at the filter inlet. Oxidation temperatures will be achieved under normal operating conditions in some applications, although in others, additional methods to initiate regeneration of a diesel particulate filter must be used. In one method, a reactant, such as diesel fuel, is introduced into an exhaust after-treatment system to generate temperature and initiate oxidation of particulate buildup in the filter. Partial or complete regeneration may occur depending on the duration of time the filter is exposed to elevated temperatures and the amount of particulate matter remaining on the filter. Partial regeneration, caused either by controlled regeneration or uncontrolled regeneration, can contribute to irregular distribution of particulate matter across the substrate of a particulate filter. Irregular or non-uniform distribution contributes to errors in estimating the amount of particulate on the filter, among other problems.
Controlled regeneration traditionally has been initiated at set intervals, such as distance traveled or time passed. Interval-based regeneration, however, has not proven to be totally effective for several reasons. First, regenerating a particulate filter with little or no particulate buildup lessens the fuel economy of the engine and unnecessarily exposes the particulate filter to destructive temperature cycles. Second, if particulate matter accumulates significantly before the next regeneration, backpressure from blockage of the exhaust flow can negatively affect engine performance. In addition, regeneration (intentional or unintentional) of a particulate filter containing large quantities of particulate buildup can become uncontrolled and potentially cause filter failure or the like. Consequently, many particulate filters regenerated on a set interval must be replaced frequently to maintain the integrity of the exhaust gas after-treatment system.
Recently, attempts have been made to estimate the amount of particulate matter accumulated in a particulate filter in order to respond more efficiently to actual particulate buildup, such as, in one widely used method, through differential pressure across a diesel particulate filter. These attempts, however, often do not account for variations in engine operating conditions, sensor noise levels, exhaust flow estimate errors, and unevenly distributed particulate accumulation. In many cases they also integrate errors over time and deviate from real soot loading conditions.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that determine a distribution of particulate matter on a particulate filter and overcome the shortcoming of the present systems.